OpenX ad platform supports ad ops staff configuring limitations in the number of times a user sees an ad in a defined session duration.

I used firebug to inspect the cookies passed on each page request and xdebug on the server. As far I can tell the impression counters use two http requests to detect fraudulent impressions. So the 2 requests set, check and clear values in the cookies - this creates a state machine that will not bump the impression counter unless a ad request is followed by and ad impression (lg.php)

The first request is the request for the ad for a particular zone (and receives cookies as part of this response)
The second request is the impression request - which asks for a 1x1 image (and receives cookies as part of this response)

 
request for ad for image banner ad
AID=1da02e891d41e726c1089be4e17d5ca8;
OABLOCK=17.1245857291;
sessionID=965ccec6008179bb5b31bea3c2cfe172
; PHPSESSID=e8629714b631a1a0e0dab302361485d5;
OASCAP=17.5; <--- the session (S) level counter for banner 17 is 5
_OASCAP[17]=1; <--- this the marker that the previous impression occured (it is set by the previous lg.php call)
_OABLOCK[17]=1245857300
 
response with banner ad
 
_OABLOCK[17]=deleted; expires=Tue, 24-Jun-2008 15:29:11 GMT; path=/
%5FOABLOCK%5B17%5D=deleted; expires=Tue, 24-Jun-2008 15:29:11 GMT; path=/
_OASCAP[17]=deleted; expires=Tue, 24-Jun-2008 15:29:11 GMT; path=/
%5FOASCAP%5B17%5D=deleted; expires=Tue, 24-Jun-2008 15:29:11 GMT; path=/
OAID=1da02e891d41e726c1089be4e17d5ca8; expires=Thu, 24-Jun-2010 15:29:12 GMT; path=/
OABLOCK=17.1245857300; expires=Fri, 24-Jul-2009 15:29:12 GMT; path=/
OASCAP=17.6; path=/ <----- bump the counter for video ad 17 to a value of 6 (OpenX can see _OASCAP=17.5 AND _OASCAP[17]=1
 
request to log (lg.php)
OAID=1da02e891d41e726c1089be4e17d5ca8;
OABLOCK=17.1245857300;
sessionID=965ccec6008179bb5b31bea3c2cfe172
PHPSESSID=e8629714b631a1a0e0dab302361485d5
OASCAP=17.6
 
response from logging (lg.php)
OAID=1da02e891d41e726c1089be4e17d5ca8; expires=Thu, 24-Jun-2010 15:29:12 GMT; path=/
_OASCAP[17]=1; path=/ <---- This is marking that the impression occured, for the next request
_OABLOCK[17]=1245857352; expires=Fri, 24-Jul-2009 15:29:12 GMT; path=/